If betta’s water become dirty with un eaten food, fish wates and unwanted chemicals, and if its not filtered time to time, bacterias populated in abundance and attach your fish. sometimes bacteria might be present in the very tap water you use. Bettas have good immune system to protect from this. however when your betta is weak and stressed, it might get attacked.
Betta may have clamped fins, lay at bottom or at surface, not eat, lose its color, turn gray, barely swim around. In more advanced cases, its body may start developing red patches, open sores and all kinds of nasty looking stuff. Different bacteria affect fish differently. Some will attack the internal organs while others prefer to infect on the skin.
Do a full water change for jars or tanks. Clean filter, change filtering system, remove any uneaten food rotting, or any dead fish!!! Isolate any bettas with symptoms if in a community tank.
You should also treat the whole tank. There is a wide variety of antibiotics available for fish. Remove carbon from your filters before you add the meds!! The carbon would otherwise absorb all the medication and you would be flushing your money down the tube.
If the sick bettas are small fries, I truly recommend using Tetracycline or Ampicillin combined with Fungus Eliminator, or whatever you find at your store look for “broad spectrum” antibiotics, though a good one is Kanamycin if you can find some. Follow manufacturer’s instructions and don’t stop the treatment until your bettas are well again. If betta is jarred, then as usual, figure out how much water your jar contains and divide the quantity of medication accordingly.